arXiv subjects

Samuel Sánchez López

Publications and source records attributed to Samuel Sánchez López.

10 recordsLinked to original sources

arXiv2026

To USR or not to USR: An effective description of two-field dynamics

Single-field models of inflection point inflation experience a phase of ultra-slow roll (USR), leading to amplified curvature perturbations. Recently, Lorenzoni et al. modified the scenario by adding a light spectator field, showing that it ruins USR by setting a floor for the first slow-roll parameter $ε$. Mixing with isocurvature perturbations still amplifies the curvature power spectrum. We offer an alternative point of view, arguing that the inflaton still undergoes effective USR. The spectator lifts the potential by a constant amount, and the inflaton perturbations follow single-field dynamics in the lifted inflection point potential, decoupling from the spectator perturbations. Mode mixing only affects the power spectrum dip, making it milder. We demonstrate this effective single-field behavior numerically for example models. Our results offer a simple way to study the models' parameter dependence.

astro-ph.CO↗

arXiv2026

Post-Newtonian Constraints on Scalar-Tensor Gravity

Solar-System constraints on a general scalar-tensor theory with generic non-minimal coupling function, non-canonical kinetic function, and scalar potential, are investigated in both the metric and Palatini formalisms. A unified post-Newtonian treatment is developed, yielding analytical expressions for the effective scalar mass, the effective gravitational coupling, and the parametrised post-Newtonian parameters $γ$ and $β$. The results show explicitly how the choice of variational principle affects the weak-field phenomenology. Comparison with Solar-System observations, primarily the Cassini bound on $γ$, indicates that the observational impact of the formalism is strongly model dependent. Generic non-minimally coupled scalar fields may satisfy significantly weaker local bounds in the Palatini case because of stronger Yukawa suppression, whereas in Brans-Dicke gravity the differences are typically small and become appreciable only in restricted regions of parameter space. For the point-particle source considered here, Palatini $f(\hat{R})$ gravity reproduces the general-relativistic exterior post-Newtonian limit, unlike metric $f(R)$ gravity.

gr-qc↗

arXiv2025

Scalar-Induced Gravitational Waves in Palatini $f(R)$ Gravity

Primordial scalar perturbations that reenter the horizon after inflation may induce a second-order Gravitational Wave spectrum with information about the primordial Universe on scales inaccessible to Cosmic Microwave Background experiments. In this work, we develop a general framework for the study of Scalar-Induced Gravitational Waves in Palatini $f(R)$ gravity, a theory that was proven to successfully realise inflation and quintessence, and consider the case of the Starobinsky-like model as an example. A regime of radiation domination with a subdominant matter component is assumed, allowing for a well-motivated perturbative approach to the gravity modifications. We calculate the kernel function and the density spectrum numerically and find accurate analytical expressions. The spectral density, which may be tested across a wide range of frequencies by upcoming Gravitational Wave experiments, is shown to differ from the General Relativity and metric $f(R)$ gravity predictions under certain conditions. We comment on previous results in the literature regarding the metric formulation and make special emphasis on the potential of these distinctive features of the spectrum to probe the two formalisms of gravity.

astro-ph.CO↗

arXiv2025

Non-Minimally Coupled Quintessence in Light of DESI

We analyze a model of quintessence governed by an exponential potential and non-minimally coupled to gravity, in light of recent datasets, including cosmic microwave background, baryon acoustic oscillations, and supernovae distance moduli observations. Mainly focusing on the Palatini formulation of gravity, a phase space analysis reveals the existence of a late-time stable de Sitter attractor as long as the non-minimal coupling constant is negative, regardless of the value of the slope of the exponential. Fitting to CMB+DESI+DESY5 data, we find strong evidence for our model over $Λ$CDM, with a Bayes factor $\log B = 5.52$. Furthermore, the data seem to prefer dynamical dark energy at $>3σ$ C.L. and a phantom crossing in the barotropic parameter of dark energy at $2-3σ$ C.L.. We find that the scalar field dynamics in the Palatini formalism provides marginally better agreement to the data compared to the metric formalism.

astro-ph.CO↗

arXiv2024

Non-oscillating Early Dark Energy and Quintessence from Alpha-Attractors

Early dark energy (EDE) is one of the most promising possibilities in order to resolve the Hubble tension: the discrepancy between early and late-Universe measurements of the Hubble constant. In this paper we propose a model of a scalar field which can explain both EDE and late Dark Energy (DE) in a joined manner without additional fine-tuning. The field features kinetic poles as with alpha-attractors. Our model provides an injection of EDE near matter-radiation equality, and redshifts away shortly after via free-fall, later refreezing to become late-time DE at the present day. Using reasonable estimates of the current constraints on EDE from the literature, we find that the parameter space is narrow but viable. As such our model is readily falsifiable. In contrast to other work in EDE, our model is non-oscillatory, which causes its decay to be faster than that of the usual oscillatory EDE, thereby achieving better agreement with observations.

astro-ph.CO↗

arXiv2023

Observable Gravitational Waves from Hyperkination in Palatini Gravity and Beyond

We consider cosmology with an inflaton scalar field with an additional quartic kinetic term. Such a theory can be motivated by Palatini $R+R^2$ modified gravity. Assuming a runaway inflaton potential, we take the Universe to become dominated by the kinetic energy density of the scalar field after inflation. Initially, the leading kinetic term is quartic and we call the corresponding period hyperkination. Subsequently, the usual quadratic kinetic term takes over and we have regular kination, until reheating. We study, both analytically and numerically, the spectrum of primordial gravitational waves generated during inflation and re-entering the horizon during the subsequent eras. We demonstrate that the spectrum is flat for modes re-entering during radiation domination and hyperkination and linear in frequency for modes re-entering during kination: kinetic domination boosts the spectrum, but hyperkination truncates its peak. As a result, the effects of the kinetic period can be extended to observable frequencies without generating excessive gravitational waves, which could otherwise destabilise the process of Big Bang Nucleosynthesis. We show that there is ample parameter space for the primordial gravitational waves to be observable in the near future. If observed, the amplitude and `knee' of the spectrum will provide valuable insights into the background theory.

gr-qc↗

arXiv2023

Explaining the Hubble tension and dark energy from alpha-attractors

A compelling unified model of dark energy and early dark energy (EDE) is presented, using a scalar field with an exponential runaway potential, in the context of alpha-attractors. The field is originally trapped at an enhanced symmetry point, subsequently thaws to become successful EDE and eventually slow-rolls to become dark energy. EDE ameliorates the observed Hubble tension.

gr-qc↗

arXiv2022

Palatini $R^2$ Quintessential Inflation

We construct a model of quintessential inflation in Palatini $R^2$ gravity employing a scalar field with a simple exponential potential and coupled to gravity with a running non-minimal coupling. At early times, the field acts as the inflaton, while later on it becomes the current dark energy. Combining the scalar sector with an ideal fluid, we study the cosmological evolution of the model from inflation all the way to dark energy domination. We interpret the results in the Einstein frame, where a coupling emerges between the fluid and the field, feeding energy from the former to the latter during the matter-dominated era. We perform a numerical scan over the parameter space and find points that align with observations for both the inflationary CMB data and the late-time behaviour. The final dark energy density emerges from an interplay between the model parameters, without requiring the extreme fine-tuning of the cosmological constant in $Λ$CDM.

gr-qc↗

arXiv2022

Modelling Quintessential Inflation in Palatini Modified Gravity

We study a model of quintessential inflation constructed in $R^2$ modified gravity with a non-minimally coupled scalar field, in the Palatini formalism. Our non-minimal inflaton field is characterised by a simple exponential potential. We find that successful quintessential inflation can be achieved with no fine-tuning on the model parameters. Predictions on the characteristics of dark energy will be tested by observations in the near future, while contrast with existing observations provides insights on the modified gravity background, such as the value of the non-minimal coupling and its running.

gr-qc↗

arXiv2021

Quintessential inflation in Palatini $f(R)$ gravity

We investigate in detail a family of quintessential inflation models in the context of $R+R^2$ Palatini modified gravity. We find that successful inflation and quintessence are obtained with an inflaton scalar potential that is approximately quadratic in inflation and inverse quartic in quintessence. We show that corrections for the kination period due to Palatini modified gravity are subdominant, while the setup does not challenge constraints on modified gravity from solar system observations and microscopic experiments, in contrast to the metric case. We obtain concrete predictions regarding primordial tensors, to be probed in the near future.

gr-qc↗